Re: [PATCH 4.14 20/65] ptrace: reintroduce usage of subjective credentials in ptrace_has_cap()

[Guenter Roeck <linux@roeck-us.net>]

On Wed, Jan 22, 2020 at 10:29:05AM +0100, Greg Kroah-Hartman wrote:
> From: Christian Brauner <christian.brauner@ubuntu.com>
> 
> commit 6b3ad6649a4c75504edeba242d3fd36b3096a57f upstream.
> 
> Commit 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat")
> introduced the ability to opt out of audit messages for accesses to various
> proc files since they are not violations of policy.  While doing so it
> somehow switched the check from ns_capable() to
> has_ns_capability{_noaudit}(). That means it switched from checking the
> subjective credentials of the task to using the objective credentials. This
> is wrong since. ptrace_has_cap() is currently only used in
> ptrace_may_access() And is used to check whether the calling task (subject)
> has the CAP_SYS_PTRACE capability in the provided user namespace to operate
> on the target task (object). According to the cred.h comments this would
> mean the subjective credentials of the calling task need to be used.
> This switches ptrace_has_cap() to use security_capable(). Because we only
> call ptrace_has_cap() in ptrace_may_access() and in there we already have a
> stable reference to the calling task's creds under rcu_read_lock() there's
> no need to go through another series of dereferences and rcu locking done
> in ns_capable{_noaudit}().
> 
> As one example where this might be particularly problematic, Jann pointed
> out that in combination with the upcoming IORING_OP_OPENAT feature, this
> bug might allow unprivileged users to bypass the capability checks while
> asynchronously opening files like /proc/*/mem, because the capability
> checks for this would be performed against kernel credentials.
> 
> To illustrate on the former point about this being exploitable: When
> io_uring creates a new context it records the subjective credentials of the
> caller. Later on, when it starts to do work it creates a kernel thread and
> registers a callback. The callback runs with kernel creds for
> ktask->real_cred and ktask->cred. To prevent this from becoming a
> full-blown 0-day io_uring will call override_cred() and override
> ktask->cred with the subjective credentials of the creator of the io_uring
> instance. With ptrace_has_cap() currently looking at ktask->real_cred this
> override will be ineffective and the caller will be able to open arbitray
> proc files as mentioned above.
> Luckily, this is currently not exploitable but will turn into a 0-day once
> IORING_OP_OPENAT{2} land in v5.6. Fix it now!
> 
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Eric Paris <eparis@redhat.com>
> Cc: stable@vger.kernel.org
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Reviewed-by: Serge Hallyn <serge@hallyn.com>
> Reviewed-by: Jann Horn <jannh@google.com>
> Fixes: 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat")
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> ---
>  kernel/ptrace.c |   15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -258,12 +258,17 @@ static int ptrace_check_attach(struct ta
>  	return ret;
>  }
>  
> -static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
> +static bool ptrace_has_cap(const struct cred *cred, struct user_namespace *ns,
> +			   unsigned int mode)
>  {
> +	int ret;
> +
>  	if (mode & PTRACE_MODE_NOAUDIT)
> -		return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
> +		ret = security_capable(cred, ns, CAP_SYS_PTRACE);
>  	else
> -		return has_ns_capability(current, ns, CAP_SYS_PTRACE);
> +		ret = security_capable(cred, ns, CAP_SYS_PTRACE);
> +
> +	return ret == 0;

This results in
	if (condition)
		do_something;
	else
		do_the_same;

Is that really correct ? The upstream patch calls security_capable()
with additional CAP_OPT_NOAUDIT vs. CAP_OPT_NONE parameter, which does
make sense. But I don't really see the benefit of the change above.

Guenter